Why Asparagus Causes Pungent Urine Odors

OK I’ll admit it, I really like asparagus. Have you ever eaten asparagus and noticed that it gives your urine a pungent odor? Or perhaps you’ve heard about the phenomenon but wondered why it doesn’t seem to affect you.

Observations of how what we eat can affect urine can be traced back through history, from the ancient Greeks. Asparagus’s potential to affect urine was described in 1735. This seems to coincide with when the British first started using fertilizers containing sulfur on crops, although this could be coincidental. And as only the Brits could put it one British men’s club is rumored to have put up a sign: “During the asparagus season, members are requested not to relieve themselves in the hat stand.”

The asparagus odiferous urine link seems to all come down to one chemical, asparagusic acid. Asparagusic acid, as it is cleverly named, seems to only be found in asparagus.

1,2-Dithiolane-4-carboxylic acid

S2(CH2)2CHCO2H

When our bodies digest the vegetable, this chemical is broken down into a group of related sulfur-containing compounds including methanethiol, dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide and dimethyl sulfone. As with many other substances that include sulfur such as rotten eggs, onions, garlic, and skunk spray these sulfur-containing molecules convey a powerful, typically unpleasant scent. These molecules are volatile, meaning that they can easily vaporize into a gaseous state at room temperature and can be detected by the human nose at very low concentrations.

Asparagusic acid, on the other hand, isn’t volatile, so asparagus itself doesn’t have the same rotten smell (asparagus probably would not be a popular vegetable if it did). But once your body converts asparagusic acid into these volatile, sulfur-bearing compounds, the distinctive aroma can be generated, and quite quickly…..in some cases, it’s been detected in the urine of people who ate asparagus just 15-30 minutes earlier.

There have been different theories put forward over the years explaining why only some people notice a smell after eating asparagus. Perhaps some people’s digestive systems don’t break down asparagusic acid into these pungent chemicals…….or perhaps not everyone can detect these smells at low levels. Depending on which study you read, between 25% and 50% of people report having pungent urine after eating asparagus.

So the issue might not be whether or not your urine is smelly, it might be whether you’re able to smell it.

Anyway I’ve long wondered about this so I thought I would share it with you. You are welcome.

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